When a heat pump type air conditioner is operated for heating, frost is sometimes deposited on the surface of an outdoor heat exchanger (a heat source side heat exchanger). When the frost closes an air duct between fins in the outdoor heat exchanger, heat exchange performance of the outdoor heat exchanger is deteriorated and a sufficient heating capacity cannot be obtained. Therefore, it is necessary to periodically determine a frosting state of the outdoor heat exchanger and remove the frost.
As a method of removing the frost, there have been known a reverse cycle defrosting operation for switching a four-way valve to a cooling operation side to remove the frost and a hot gas bypass defrosting operation for providing a hot gas bypass circuit bypassed from a compressor discharge side and including an on-off valve, connecting the circuit to an outdoor heat exchanger inlet side, and feeding a part of a compressor discharge gas refrigerant to an outdoor heat exchanger to remove the frost.
As a conventional technique for switching the hot gas bypass defrosting operation and the reverse cycle defrosting operation to perform defrosting operation, for example, there is a technique described in Patent Literature 1 (JP-A-2008-96033). Patent Literature 1 describes an invention for, when detecting frosting on an outdoor heat exchanger, switching a four-way valve to perform the reverse cycle defrosting operation and, when a pipe heat storage amount serving as a defrosting heat source detected by heat-storage-amount detecting means is equal to or smaller than a set value, switching the four-way valve to a regular cycle side and opening a hot gas bypass on-off valve to perform the hot gas bypass defrosting operation.
As another conventional technique, there is a technique described in Patent Literature 2 (JP-A-2011-144960). Patent Literature 2 describes an invention for, in an air conditioner including two defrosting operation systems of defrosting operation of a hot gas bypass system and defrosting operation of a reverse (reverse cycle) system, carrying out defrosting by the reverse system when the number of revolutions of a compressor is equal to or larger than a predetermined number of revolutions and increasing the number of revolutions of the compressor and performing the defrosting operation according to the hot gas bypass system when the number of revolutions of the compressor is smaller than the predetermined number of revolutions.